The dynamic focus electron gun shown in FIG. 1 comprises a tripolar section consisting of a cathode K, a control grid G1 and a screen grid G2, and a main lens consisting of a static focus electrode G3s, a dynamic focus electrode G3f and an anode G4. Further, blades BV' and BH' are provided for forming quadrupole lenses above and below beam passing holes 3Hs disposed at the beam exiting side of a focus electrode G3s to which a static potential focus voltage is supplied and at opposite sides of beam passing holes 3Hf disposed at the beam entrance side of a dynamic electrode G3f to which a dynamic potential focus voltage is supplied.
Unlike other ordinary dynamic electron guns, the above described dynamic electron gun establishes dynamic electric fields having four poles to form dynamic focus lenses of different chracteristics in the vcertical and horizontal directions. As is well known, such a dynamic focus electron gun properly controls the electron beams by means of the quadrupole lenses which provide variable intensities depending on the positions at which the electron beams strike a screen, so that uniform beam spots are formed over the whole face of the screen.
But the electron gun having the constitution described above has problems as described below. The blades BH', BV' forming the quad polar lenses are exposed to the outside of the electrodes G3s, G5s, and therefore, the assembling process becomes very difficult. The exposed blades BH', BV' are also installed in a mutually crossing manner, and therefore, arcing between the two blades BH', BV' an occur.
This can be described in detail in the following way for a more thorough understanding. Generally, if the electrodes are to be precisely installed, they should be secured with spacers inserted between them, but in the above described conventional electron gun, the exposure of the blades make it impossible to insert a spacer between the static potential focus electrode G3s and the dynamic potential focus electrode G3f during assembly, with the result that the gaps between the focus electrodes G3s and the focus electrode G3f can not be easily adjusted. Further, the blades BV', BH' having sharp edges intersect each other, and therefore, and electric discharge can occur between the focus electrode G3s and the dynamic electrode G3Fwhen there is a high potential difference between the two electrodes.